Differential OFDM using multiple receiver antennas

ABSTRACT

Systems and methods for optimally receiving differential encoded OFDM signals via multiple antennas. These techniques may exploit spatial diversity without knowledge of channel characteristics. Further systems and methods are provided for exploiting frequency diversity within an OFDM burst where differentially encoded symbols are repeated to assure optimal performance. The output of differential decoding systems may also be used to provide soft decision values for individual bits of multibit symbols to facilitate use of bitwise channel decoding systems.

BACKGROUND OF THE INVENTION

The present invention relates to data communication and moreparticularly to data communication over substantially orthogonalfrequency channels.

Orthogonal frequency division multiplexing (OFDM) systems offersignificant advantages in many real-world communication systems,particularly in environments where multipath effects impair performance.OFDM divides the available spectrum within a channel into narrowsubchannels. In a given so-called “burst”, each subchannel transmits onedata symbol. Each subchannel, therefore operates at a very low data ratecompared to the channel as a whole. To achieve transmission inorthogonal subchannels, a burst of frequency domain symbols areconverted to the time domain by an IFFT procedure. To assure thatorthogonality is maintained in dispersive channels, a cyclic prefix isadded to the resulting time domain sequence. The cyclic prefix is aduplicate of the last portion of the time domain sequence that isappended to its beginning. To assure orthogonality, the cyclic prefixshould be as long as the duration of the impulse response of thechannel.

To maximize the performance of an OFDM system, it is desirable that theresponse of the channel be known at the receiver end of the link. Toprovide the receiver with knowledge of the channel response, thetransmitter typically includes training symbols as part of the frequencydomain sequence. The training symbols have known values when transmittedand their values as received indicate the channel response. The numberof training symbols should generally be greater than the length of theduration of the impulse response of the channel.

The use of training symbols or transmission of channel responseinformation as data takes away from the data carrying capacity of thelink. Furthermore, the number of symbols used for training in a givenburst does not decrease when smaller bursts must be used, e.g., toreduce latency for voice traffic or decrease sensitivity to phase noise.For systems that employ short bursts, the efficiency loss due totraining is even greater.

The discussion up until now has assumed a point to point link. However,the loss of data carrying capacity due to channel training is greatlycompounded in point to multipoint systems where channel capacity isshared among many nodes. In a point to multipoint system, the channelresponse is different for every combination of access point and remotestation. Each separate channel response must be learned, representing agreat loss of efficiency.

One way of using OFDM in dispersive channels without the use of channeltraining is to apply differential coding or modulation to the frequencydomain symbols. Such a differential scheme encodes data as phasedifferences between frequency domain symbols. Channel magnitude responsethus does not corrupt data transmission because the receiver system doesnot take received magnitude into account in estimating the transmitteddata. Phase magnitude response also does not corrupt data transmissionbecause any phase difference applied by the channel is effectivelysubtracted out as a part of the differential decoding process.

Another useful communication technique is the use of multiple receptionantennas. The resulting spatial diversity may be exploited to amelioratethe effects of interference. IMPROVED SYSTEM FOR INTERFERENCECANCELLATION, U.S. application Ser. No. 09/234,629, filed on Jan. 21,1999, the contents of which are herein incorporated by reference,discloses the application of spatial diversity to an OFDM system toameliorate interference. The techniques disclosed there are heavilyreliant on knowledge of channel characteristics. It would be highlydesirable to optimally combine input from multiple antennas withoutknowledge of channel characteristics.

SUMMARY OF THE INVENTION

Systems and methods for optimally receiving differential encoded OFDMsignals via multiple antennas are provided by virtue of the presentinvention. These techniques may optimally exploit spatial diversitywithout knowledge of channel characteristics. The present inventionfurther provides systems and methods for exploiting frequency diversitywithin an OFDM burst where differentially encoded symbols are repeatedto assure optimal performance. The output of differential decodingsystems may also be used to provide soft decision values for individualbits of multibit symbols to facilitate use of bitwise channel decodingsystems.

According to a first aspect of the present invention, a system forreceiving OFDM signals via multiple outputs of a channel includes: aplurality of transform processors, each transform processor convertingtime domain symbols received via one of the channel outputs to frequencydomain symbols, a plurality of differential processors, eachdifferential processor obtaining frequency domain symbols from one ofthe plurality of transform processors and removing differential encodingor modulation from the frequency domain symbols.

According to a second aspect of the present invention, a system fortransmitting OFDM signals via a channel includes: a differentialprocessor that differentially encodes frequency domain symbols to betransmitted, and a transform processor that transforms bursts of thefrequency domain symbols into bursts of time domain symbols wherein atleast selected ones of the frequency domain symbols are repeated withinthe bursts.

According to a third aspect of the present invention, a system forreceiving OFDM signals via a channel including: a transform processorthat transforms a burst of time domain symbols into a burst of frequencydomain symbols, and a plurality of differential processors, each of thedifferential processors obtaining as input frequency domain symbols froma corresponding segment of the burst, the differential processorsdifferentially decoding the frequency domain symbols.

A further understanding of the nature and advantages of the inventionsherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an OFDM receiver system employing multiple antennasaccording to one embodiment of the present invention.

FIGS. 2A-2B are symbol constellation diagrams useful in describingdevelopment of bit-wise cost metric values for phase shift keying symbolsets.

FIG. 3 depicts an OFDM transmitter system employing repetition codingaccording to one embodiment of the present invention.

FIG. 4 depicts an OFDM receiver system employing repetition codingaccording to one embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

OFDM Communications

In one embodiment, the present invention may be implemented in thecontext of an OFDM communication system. The abbreviation “OFDM” refersto Orthogonal Frequency Division Multiplexing. In OFDM, the availablebandwidth is effectively divided into a plurality of subchannels thatare orthogonal in the frequency domain. During a given symbol period,the transmitter transmits a symbol in each subchannel. To create thetransmitted time domain signal corresponding to all of the subchannels,an FFT is applied to a series of frequency domain symbols to besimultaneously transmitted, a “burst.” The resulting series of timedomain symbols is augmented with a cyclic prefix prior to transmission.The cyclic prefix addition process can be characterized by theexpression:[z  (1)  …  z  (N)]^(T) ↦ [z  (N − v + 1)  …  z  (N)  z  (1)  …  z  (N)]^(T)

On the receive end, the cyclic prefix is removed from the received timedomain symbols. An IFFT is then applied to recover the simultaneouslytransmitted frequency domain symbols. The cyclic prefix has length vwhere v is greater than or equal to a duration of the impulse responseof the channel and assures orthogonality of the frequency domainsubchannels.

There are other ways of creating transmitted bursts of symbols inorthogonal channels or substantially orthogonal channels including,e.g., use of the Hilbert transform, use of the wavelet transform, usinga batch of frequency upconverters in combination with a filter bank,etc. Wherever the term OFDM is used, it will be understood that thisterm includes all alternative methods of simultaneously communicating aburst of symbols in orthogonal or substantially orthogonal subchannelsdefined by procedures performed on a time domain sequence. The termfrequency domain should be understood to refer to any domain that isdivided into such orthogonal or substantially orthogonal subchannels.

Differential OFDM

Phase differences between complex frequency domain OFDM symbols may beused to communicate data. For example, consider a constellation ofpossible symbols that consists of four symbols all having the samemagnitude and four equally distributed phases, π/4, 3π/4, 5π/4, and7π/4. There are four possible phase differences between symbols of apair, 0, π, −π/2, and π/2. Thus successive pairings of symbolscommunicate 2 bits of data in such a system. In one representative typeof differential OFDM system, data is communicated as phase differencesbetween corresponding frequency domain symbols of successive bursts. Inanother representative type of differential OFDM system, data iscommunicated as phase differences between successive frequency domainsymbols within the same burst. These are merely exemplary, and thepresent invention may be applied to processing phase differences betweenany set of pairs of frequency domain symbols.

The present invention also contemplates application to DifferentialAmplitude and Phase Shift Keying (DAPSK) systems. In a DAPSK system, thetransmitted symbol constellation consists of two or more concentric PSKrings. Two or more bits are communicated by the phase differencesbetween corresponding symbols of successive bursts. At least one bit iscommunicated by the amplitude differences or ratios betweencorresponding symbols of successive bursts. A DAPSK system is disclosedin Rohling et al., “Differential Amplitude Phase Shift Keying—A NewModulation Method for DTVB,” International Broadcasting convention Sep.14-18, 1995, (Conf. Pub. No. 713), the contents of which are hereinincorporated by reference.

Differential OFDM is advantageous in that the data received does notdepend on the characteristics of the channel. Consider differential datad(1) which is communicated by the phase difference between OFDMfrequency domain symbols in bin 1 of successive bursts, z(1, 1) andz(1,2) where z(n,k) is the data symbol transmitted in subchannel n inburst k. The OFDM frequency domain symbols recovered by the receiverwill be x(1,1) and x(1,2) where x(n,k) is the data symbol received insubchannel n in burst k. The transmitted data is then recovered byobtaining the phase of x*(1,1)x(1,2). If the channel response at bin 1is h(1), then the receiver effectively obtains:

x*(1,1)x(1,2)=[h(1)z(1,1)]*[h(1)z(1,2)]=z*(1,1)h*(1)h(1)z(1,2)=|h(1)|²z*(1,1)z(1,2)

The phase of this expression communicates data and is unaffected by thechannel response since the channel response is assumed to besubstantially the same at the same frequency for two successive burstsor at adjacent frequencies within the same burst. Thus it is unnecessaryto know the channel characteristics to accurately recover thetransmitted data. (In a DAPSK application, further data would beobtained by amplitude ratios.)

This technique is particularly advantageous in certain applications. Forexample, where one is employing a sparse symbol constellation, e.g., 4-8symbols, there is little or no increase in required signal to noiseratio due to unavailability of the magnitude component for communicatinginformation. For four symbols, there is no increase in required signalto noise ratio. For eight symbols, there is some increase.

Even for denser constellations, the ability to communicate accuratelywithout the use of training symbols may compensate for any increase inrequired signal to noise ratio. To accurately characterize the channel,at least v training symbols are required in a single burst where v is aduration of the channel impulse response. The training symbols havepredetermined values and typically carry no data. Where channel impulseduration is a significant fraction of burst length, the loss of datacarrying capacity to training becomes important. An example of thissituation would be an OFDM system with severe phase noise carrying voicetraffic across a dispersive fading channel. The system designer isforced to maintain a relatively short burst length to maintain lowlatency time and low sensitivity to phase noise, yet a large number oftraining symbols are necessary to repeatedly characterize the dispersivechannel. The present invention offers a solution for this application bysuccessfully communicating data without full channel knowledge.

In other applications, the collection of channel training informationmay be infeasible for other reasons. In a point to multipoint system,separate channels will exist between each of many remote transceiversand a central access point although the central access point willcommunicate with only one transceiver at a time. Each separate channelthus would be trained independently. The present invention may beapplied here to free up capacity. The effect is the same for any systemwhere access to a common transmission medium is shared.

Furthermore, shared access systems are often designed so thattransceivers desiring access to the medium will first transmit one ormore access request bursts. The central access point receiving an accessrequest may not have recently received anything transmitted by theaccess point so any past training information would be too stale foruse.

Differential OFDM with Multiple Receiver Antennas

FIG. 1 depicts an OFDM receiver system 100 employing multiple antennasaccording to one embodiment of the present invention. Receiver system100 collects signals from a plurality of antennas 102. In FIG. 1, twoantennas are shown, although any number of antennas may be used. Manycomponents depicted in FIG. 1 are duplicated for each antenna.

Each antenna 102 is coupled to an RF/IF system 104 which performsinitial analog filtering and amplification prior to downconversion to anintermediate frequency (IF) where further filtering and signalconditioning may be performed. The signal is then converted to basebandfor input to an analog to digital converter 106. Alternatively, analogto digital conversion may occur at the IF. The next depicted stage is anFFT processor 108 that removes the cyclic prefix from N+ν long timedomain symbol bursts and then applies the FFT to recover N frequencydomain symbols for each successive OFDM burst.

For each antenna 102, a differential decoding stage 110 recovers thedifferentially encoded data based on the frequency domain symbols outputby the corresponding FFT processor 108. When data is encoded as phasedifferences between corresponding frequency domain symbols of successivebursts, each differential decoding stage 110 finds a detection symbol:

a _(i)(n,k)=x* _(i)(n,k)x _(i)(n,k+1).

When differential data is encoded as phase differences between adjacentsymbols, differential decoder 110 finds:

a _(i)(n,k)=x* _(i)(n,k),x _(i)(n+1,k)

where x refers to a received frequency domain symbol, i identifies aparticular antenna, n identifies a frequency domain symbol positionwithin a burst, and k identifies a particular burst.

A combination element 112 combines the detection symbols obtained viathe multiple antennas to form a combined detection symbol estimate. Inone embodiment, combination element 112 finds the combined detectionsymbol to be:${a\left( {n,k} \right)} = {\sum\limits_{i = 1}^{M_{R}}\quad {a_{i}\left( {n,k} \right)}}$

The recovered phase difference value is then:

{circumflex over (z)}(n,k)=∠a(n,k).

In one embodiment, differential encoding/modulation is combined withchannel coding techniques such as trellis coding or convolutionalcoding. It is advantageous to use a trellis decoder that requires softdecision or cost metric values as input. A cost metric value processor114 calculates the cost metric values to be:

c(n,k)=|a(n,k)|² |{circumflex over (z)}(n,k)−{overscore (z)}(n,k)|²

where

{overscore (z)}(n,k) is the nearest ideal phase difference value to{circumflex over (z)}(n,k). For example, the ideal phase differencevalues for QPSK would be {0, π/2, −π/2, π}.

The term |a(n,k)|² serves as a confidence value which weights phasedifferences received by the various antennas according to theirassociated symbol magnitudes. The cost metric values are then the inputto a trellis decoder 116.

It may be advantageous to substitute a less complex Viterbi decoder fortrellis decoder 116. A Viterbi decoder, however, requires cost metricvalues for individual bits rather than individual symbols. The processof developing bitwise cost metric values based on the phase differencedata will be described in reference to FIGS. 2A-2B which depict apossible set of four ideal symbols 200 as would be output bydifferential decoding stages 110 in the absence of noise, interference,and channel distortion as well as a an actual received symbol 202

Ideal symbol set 200 assumes that the frequency domain symbols arechosen according to a differential quadrature phase shift key (DQPSK)scheme, although of course this scheme is merely representative. Allfour symbols have the same magnitude but different phases. Each symbolrepresents two bits as labeled. The cost metric values are givenindependently for each bit.

The cost metric value on a bit-wise basis may be given by:

c(n,k,m)=|a(n,k)|² |d ₀(n,k,m)−d ₁(n,k,m)|²sign[d ₀ −d ₁]

where n identifies a frequency domain position, k identifies a burstnumber, and m identifies a particular bit position. The term d₀ (n,k,m)is the angular difference between the polar coordinates of the receivedsymbol and the polar coordinates of the nearest (in angle) ideal symbolhaving a 0 at bit position m. Similarly, d₁(n,k,m) is the angulardifference between the polar coordinates of the received symbol and thepolar coordinates of the nearest (in angle) ideal symbol having a 1 atbit position m. FIG. 2A depicts d₀ and d₁ for the most significant bitand FIG. 2B depicts these quantities for the least significant bit.

According to the present invention, one may also exploit differentialcoding in conjunction with frequency diversity rather than spatialdiversity. Differential coding is applied to frequency domain symbolswhich are repeated within the frequency domain structure of an OFDMburst. Thus the frequency domain structure of the OFDM burst may includetwo or more repeated segments. At the receiver end, these repeatedsegments may be processed as if they had been received via multipleantennas as in the system of FIG. 1.

FIG. 3 depicts an OFDM transmitter system 300 employing repetitioncoding according to one embodiment of the present invention. Adifferential coder 302 applies differential coding to an input datastream. The data may of course may be the output of other encodingprocesses. The output of differential coder 302 is a series of burstsegments, each burst segment including OFDM frequency domain symbols.Differential coder 302 may, e.g., encode the data as phase differencesbetween symbols adjacent in the frequency domain, or as phasedifferences between corresponding symbols in successively generatedburst segments.

To implement repetition coding, each burst segment is repealed by arepetition coder 304 so that it fills the burst length used bytransmitter system 300. The burst segment may be repeated twice, or 3 ormore times. This provides frequency diversity. Of course, the frequencydiversity typically comes at the expense of capacity. An IFFT processor306 performs the IFFT on successive frequency domain bursts to obtainsuccessive time domain bursts and adds a cyclic prefix to each timedomain burst to assure orthogonalization. An RF/IF system 308 convertsthe baseband symbol stream to an IF analog signal and then to an RFsignal ready for transmission via an antenna 310.

FIG. 4 depicts an OFDM receiver system 400 taking advantage ofrepetition coding according to one embodiment of the present invention.An antenna 402 is coupled to an RF/IF system 404 which performs initialanalog filtering and amplification prior to downconversion to anintermediate frequency (IF) where further filtering and signalconditioning may be performed. The signal is then converted to basebandfor input to an analog to digital converter 406. FFT processor 408removes the cyclic prefix from N+ν long time domain symbol bursts andthen applies the FFT to recover N frequency domain symbols for eachsuccessive OFDM burst.

The output of FFT processor 408 is a series of frequency domain bursts.Each burst includes two or more repeated segments. The repeated segmentsmay be combined in the same manner as the frequency domain symbolsreceived via multiple antennas in FIG. 1. For each segment, there is aseparate differential decoding stage 410. When data is encoded as phasedifferences between corresponding frequency domain symbols of successivebursts, differential decoding stage 410 finds:

a _(i)(n,k)=x*_(i)(n,k)x _(i)(n,k+1).

When differential data is encoded/modulated as phase differences betweenadjacent symbols, each differential decoding stage 410 finds:

a _(i)(n,k)=x* _(i)(n,k),x _(i)(n+1,k)

where x refers to a received frequency domain symbol, i identifies aparticular segment rather than a particular antenna, n identifies afrequency domain symbol position within a burst, and k identifies aparticular burst.

A combination element 412 then operates in the same way as combinationelement 112 to combine the phase difference values developed fromdifferent burst segments. A cost metric value processor 414 thendevelops soft cost metric values for input to a trellis decoder 416using the techniques described in reference to cost metric valueprocessor 114. The cost metric values may be developed on either a bitby bit or symbol by symbol basis.

Repetition coding may also be employed with multiple antennas to provideboth frequency diversity and spatial diversity. The combinationtechniques are the same as described in reference to FIGS. 3-4.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims and their full scope of equivalents. Forexample, all formulas given above are merely representative ofprocedures that may be used. Antennas are merely representative ofchannel inputs and outputs and the present invention may also be appliedto wireline systems. Functionality may be added or deleted from FIGS. 1and 3-4 and operations may be interchanged among functional blocks. Allpublications, patents, and patent applications cited herein are herebyincorporated by reference.

What is claimed is:
 1. A system for receiving OFDM signals via multipleoutputs of a channel, comprising: a plurality of transform processors,each transform processor converting time domain symbols received via oneof said channel outputs to frequency domain symbols; a plurality ofdifferential processors, each differential processor obtaining frequencydomain symbols from one of said plurality of transform processors andremoving differential encoding from said frequency domain symbolswherein differences between pairs of frequency domain symbols representmultiple bits; a combination processor that combines output of each ofsaid differential processors to estimate transmitted data withoutestimation of said channel; and a cost metric value processor thatresponsive to output of said combination processor estimates softdecision values for each of said multiple bits.
 2. The system of claim 1wherein said combination processor sums output of said differentialprocessors to estimate transmitted data.
 3. The system of claim 1wherein said differential processors estimate phase differences betweencorresponding frequency domain symbols in successive bursts of frequencydomain symbols.
 4. The system of claim 1 wherein said differentialprocessors estimate phase differences between successive frequencydomain symbols within a burst.
 5. A system for receiving OFDM signalsvia a channel comprising: a transform processor that transforms a burstof time domain symbols received via a single antenna into a burst offrequency domain symbols; a plurality of differential processors, eachof said differential processors obtaining as input frequency domainsymbols from a corresponding segment of said burst, said differentialprocessors differentially decoding said frequency domain symbols whereindifferences between pairs of frequency domain symbols represent multiplebits; a combination processor that combines output of said differentialprocessors to estimate transmitted data; and a cost metric valueprocessor that responsive to outputs of said differential processorsestimates soft decision values for each of said multiple bits.
 6. Thesystem of claim 5 wherein said combination processor averages output ofsaid differential processors to estimate transmitted data.
 7. The systemof claim 5 wherein said differential processors estimate phasedifferences between corresponding frequency domain symbols in successivebursts of frequency domain symbols.
 8. The system of claim 5 whereinsaid differential processors estimate phase differences betweensuccessive frequency domain symbols within a burst.
 9. A method forreceiving OFDM signals via multiple outputs of a channel, said methodcomprising: converting time domain symbols received via each of saidchannel outputs to frequency domain symbols; for each of said channeloutputs, differentially decoding said frequency domain symbols to obtaindifferentially decoded frequency domain symbols wherein differencesbetween pairs of said frequency domain symbols represent multiple bits;and combining said differentially decoded frequency domain symbolsobtained for each of said channel outputs to estimate transmitted datawithout estimation of said channel; and wherein combining comprisesestimating soft decision values for each of said multiple bits.
 10. Themethod of claim 9 wherein combining comprises summing differentiallydecoded frequency domain symbols obtained for each of said channeloutputs.
 11. The method of claim 9 wherein differentially decodingcomprises estimating phase differences between successive frequencydomain symbols within a burst.
 12. The method of claim 9 whereindifferentially decoding comprises estimating phase differences betweencorresponding frequency domain symbols in successive bursts of frequencydomain symbols.
 13. A method for receiving OFDM signals via a channel,said method comprising: converting a burst of time domain symbolsreceived via a single antenna into a burst of frequency domain symbols;differentially decoding said frequency domain symbols whereindifferential decoding is performed in parallel for multiple frequencydomain segments of said burst, wherein differences between pairs offrequency domain symbols represent multiple bits; and combining resultsof said differential decoding for said multiple frequency domainsegments to estimate transmitted data; and wherein combining comprisesestimating soft decision values for said multiple bits.
 14. Apparatusfor receiving OFDM signals via multiple outputs of a channel, saidapparatus comprising: means for converting time domain symbols receivedvia each of said channel outputs to frequency domain symbols; means for,for each of said channel outputs, differentially decoding said frequencydomain symbols to obtain differentially decoded frequency domainsymbols, wherein differences between pairs of frequency domain symbolsrepresent multiple bits; and means for combining said differentiallydecoded frequency domain symbols obtained for each of said channeloutputs to estimate transmitted data without estimation of said channel;and wherein said means for combining comprises means for estimating softdecision values for said multiple bits.
 15. Apparatus for receiving OFDMsignals via a channel, said apparatus comprising: means for converting aburst of time domain symbols received via a single antenna into a burstof frequency domain symbols; means for differentially decoding saidfrequency domain symbols wherein differential decoding is performed inparallel for multiple frequency domain segments of said burst; whereindifferences between pairs of frequency domain symbols represent multiplebits; and means for combining results of said differential decoding forsaid multiple frequency domain segments to estimate transmitted data;and wherein said means for combining comprises means for estimating softdecision values for said multiple bits.